Field termination plate

ABSTRACT

Field termination plates for cylindrical electron analyzers are provided wherein the plates are constructed of an insulative material coated on the interior surface with a high resistance, electrically conducting coating. Spaced concentric rings of relatively high conductivity material in electrical contact with said coating are provided; the rings providing equi-potential regions on the plates, thereby minimizing field fringing near the ends of the cylindrical tube electron analyzer.

United States Patent n91 Palmberg [54] FIELD TERMINATION PLATE [75] Inventor: Paul W. Palmberg, Minneapolis, Minn. v

[73] Assignee: Physical Electronics, Inc., Edina,

' Minn.

[22] Filed: Aug. 27, 1971 [21] Appl.No.: 175,633

[52] US. Cl. ..250/49.5 C, 250/419 ME, 313/85 51 int. Cl. ..B0ld 59/44 58 Field of Search ..250/49.5 c, 105, 250/419 ME; 313/352, 356, 85, 63, 83

[56] References Cited UNITED STATES PATENTS 2,506,659 5/1950 Bezy ..3l3/83 3,407,323 10/1968 Hand ..250/4l.9 ME

[4 1 May 22, 1973 9/1950 Trump etal 7/1971 .313/63 l-lelmer ..250/49.5 AE

Primary ExaminerJames W. Lawrence Assistant Examiner-Harold A. Dixon Attorney-Schroeder, Siegfried, Rayan & Vidas [57] ABSTRACT Field termination plates for cylindrical electron analyzers are provided wherein the plates are constructed of an insulative material coated on the interior surface with a high resistance, electrically conducting coating. Spaced concentric rings of relatively high conductivity material in electrical contact with said coating are provided; the rings providing equi-potential regions on the plates, thereby minimizing field fringing near the ends of the cylindrical tube electron analyzer.

14 Claims, 4 Drawing Figures FIELD TERMINATION PLATE The present invention is directed to the field of electrostatic analyzers generally, and is more specifically directed to the field of cylindrical mirror analyzers formed of concentric cylinders used to analyze the energy distribution of charged particles such as electrons. While devices of this type are useful in a variety of analytical schemes, the invention will be described with particularity as used in an electron analyzer for use in Auger spectroscopy.

It is important that the electric field between the concentric cylinders be fringe-free (i.e. E=k/r where r radius from the axis of the cylinders) throughout the portion of the analyzer that affects the analysis of the electrons. One manner in which this can be accomplished is to extend each end of the concentric cylinders well beyond the region where the electrons actually pass through the analyzer, so that field fringing near the ends of the cylinders will not interfere with the analysis. This extension of the ends of the concentric cylinders avoids to a large degree the problems of fringing. However, such a construction is undesirable for a number of reasons. In Auger electron spectroscopy, the entire apparatus must be contained within a very high vacuum, and the excess amount of space consumed by having elongated tubes would be a serious problem. Additionally, with elongated tubes there would be problems in positioning the sample to be analyzed, an activating source such'as an electron gun, and the detecting apparatus.

The copending application of Bohn et al., Ser. No. 68,983, filed Sept. 2, 1970, for AUGER ELECTRON SPECTROSCOPY, assigned to the same assignee as the present invention, is incorporated herein by reference. In this previously filed patent application, a solution was offered for the problem of fringing. The solution in the Bohn et al. application is the use of ceramic washers for spacing the concentric tubes at each end thereof; the ceramic washers having over the inner face thereof a conductive coating of high resistivity. This coating over the surface of the ceramic washer provided a marked improvement in the avoiding of fringing near the ends of the concentric tubes. While considerable improvement over the prior art was achieved, the results are not as completely fringe free as is desirable.

l have found that fringing can be substantially eliminated in concentric tube analyzers if one utilizes insulative spacing washers at each end of the concentric tube assembly having a high resistance coating in accordance with the Bohn et al disclosure; and, in addition, having means for compensating for variations in the resistivity of the coating. In accordance with my invention, the effect of anomalous regions in the high resistance coating that provide for nonuniformity in the field patterns is substantially eliminated by having a series of concentric rings of relatively high conductivity material positioned across the face of the ceramic disc and in electrical contact with the high resistance film. Each of these rings provides a region of equi-potential along a given radius and thus effectively reduces fringing. The invention will be best understood from a study of the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of an interior face of a field termination plate in accordance with the invention;

FIG. 2 is a cross-sectional view along lines 2-2 of FIG. 1;

FIG. 3 is a schematic view, partly in section, of a cylindrical electron analyzer incorporating the invention; and

FIG. 4 is an alternate form of the invention in crosssectional view including a portion of the cylindrical tubes of the analyzer.

In the description which follows, like parts will be given the same numerical designations in each of the several views.

Turning first to FIG. 3, there is illustrated in schematic and in cross-sectional view a concentric cylinder electron analyzer wherein the electron gun used to excite the sample to be analyzed is located interior of the innermost cylinder and is intermediate the openings in the inner cylinder through which the electrons pass in being analyzed. This construction is described in greater detail in the Bohn et al. application referred to above. 1

The analyzer consists of an outer cylinder 11 and inner cylinder 12 which are concentric with one another. These tubes 11 and 12 are held in spaced relationship to one another by electrically insulating washers 13 and 14 on each end of the tube. Tubes 11 and 12 are desirably made of a conductive metal such as stainless steel, while the spacers 13 and 14 can be alumina or other ceramic. At spaced positions along tube 12 are openings 15 and 16, substantially entirely around the periphery of tube 12. Narrow strips 17 are retained to support the central portion of the tube. Across openings 15 and 16 there is provided a wire screen of about 100 lines per inch, which aids in electrical shielding but permits the electrons to pass therethrough as the screen is about percent transparent.

As previously indicated, an electron gun 18 is coaxially located within the region intermediate openings 15 and 16 of tube 12. Alternatively, the electron gun may be external to the tubes in the so-called grazing incidence position relative to the sample being analyzed. The path of electrons emitted by the electron gun is designated by arrow 19 and is directed to atarget material 20 which is positioned by means, not shown, near the opening of tube 12. Target 20 will, of course, be the sample being analyzed.

When target 20 is bombarded with electrons from the electron gun, secondary electrons are emitted from the target material. Some of the emitted electrons are known as Auger electrons. Some of them pass along a path designated 21 through the space intermediate cylinders 11 and 12. Field applying means and analysis means are illustrated in a general way by the circuitry associated with FIG. 3, with the inner tube 12 being at ground potential while tube 11 is maintained at some elevated negative value such as 1,000 volts. The electrons passing through opening 15 are deflected along the path generally indicated as 21 and, depending upon their energy, pass through opening 16 and then through exit aperture 22, which is positioned at the point of minimum trace width for the electrons of interest. The, analyzed electrons which have passed through the analyzer are then detected by some means such as electronmultiplier 23 and the signal resulting therefrom treated as is known in the art.

well known in the art.

In an arrangement such as described with regard to FIG. 3, the field gradient between cylinder 11 and cylinder 12 is fringe-free in the intermediate portions along the length of the tubes, but tends to have fringing near the ends due to the termination of the tubes 11 and 12. As previously noted, one solution to this problem has been proposed in the copending application of Bohn et'al. wherein a high resistance cermet coating has been applied across the interior surface of ceramic spacers 13 and 14 to diminish the tendency for field fringing. The nature of the cermet material, as well as other resistive material which can be used, is such that complete uniformity of resistivity across the surface of the spacers is difficult to achieve. If low resistance paths exist across the plates 13 and 14, some degree of fringing is the inevitable result.

In accordance with my invention, I have found that I can markedly decrease the effect of any nonuniformity of the resistance coating on the surfaces of the ceramic spaces 13 and 14. Turning to FIG. 1, there is illustrated in front elevational view a ceramic spacer in accordance with my invention. FIG. 2 is a crosssectional view along lines 2-2 of FIG. 1. The spacer is generally designated 24 and may be formed of any of a variety of insulative materials. As the element is to be used under high vacuum conditions where thermal means are often utilized to outgas the entire system, it is desirable that spacer 24 have good temperature stability and low gas retaining qualities. I have found that it is advantageous to use a ceramic material such as alumina or the quartz form of silicon dioxide for the spacers. A typical size for the tubes comprising the analyzer is about 1 inch [.D. for the inner cylinder and about 2 and 1/2 inches ID. for the outer cylinder. As best seen in FIG. 2, the washer has been provided with a shoulder 25 to aid in the mounting of the washer inthe tube assembly.

As points of equi-potential, my invention provides a series of concentric rings of narrow thickness and width spaced uniformly across the surface of the spacer. These rings are numbered 26 through 30, respectively. The rings are desirably of about 0.005 inches in width, although wider or narrower rings can be provided. The number of rings can also be varied. While a variety of materials are useful for forming the rings, I have found that a gold-chromium ring is particularly useful in the invention due to its compatibility with other materials that are used in the invention. The chief criterion that must be met is that the rings should be relatively high in electrical conductivity when compared with the resistive material that will be described below. They also should be of a material which remains essentially stable under conditions of use so that it does not react with the high resistance material or with the ring proper.

Rings 26 through 30 can be produced in a variety of manners. I prefer to vacuum sputter the goldchromium over the interior surface of the ceramic and then use photolithographic masking and etching to produce the final rings. The thickness of the deposited material is not critical, but should be thick enough to give a relatively low resistance path when compared to the high resistance material.

Over the surface of rings 26 through 30 I then deposit a high resistivity material in a manner such as to produce electrical contact between the material and each of the rings. The preferred resistive material is a cermet, although this is not required. I have found that the thermal stability of a cermet, insofar as its resistivity is concerned, is desirable in the invention. A suitable cermet is that formed by vacuum sputtered alumina and nickel metal to form a thin layer having the desired resistance. A resistance of approximately 30 megohms from the center to the outer edge of the plate is preferred, although resistances from 10 to megohms are suitable.

Alternatively, one may deposit the cermet resistance material first and then superimpose the rings 26 through 30 over the outer surface of the cermet. However, it is preferred to deposit the ring members heneath the cermet coating.

It should be appreciated that rings 26 and 30 are in electrical contact with tubes 11 and 12, respectively.

It should now be apparent to the reader that through the use of the rings 26 through 30 a plurality of equipotential regions are provided on the surfaces of the ceramic spacer. Any local variation in the resistivity of the coating 31 is noncumulative, as the individual rings provide a leveling effect to the resistance path. At each ring, the potential will be the same around the entire periphery of the disc at that radius.

In FIG. 4, there is shown a modification of the invention as shown in FIGS. 1 and 2, wherein a spacer element 32 has a generally hollow frusto-conical configuration. Element 32 may be advantageously made of alumina, as is the case with the earlier description. This form of the invention has been prepared in a manner analogous to the description given above, so that the interior surface thereof has a plurality of ring members 26', 27', 28', 29' and 30', corresponding in construction and positioning to the rings 26 through 30 of FIGS. 1 and 2. When viewed from the inside surface, the appearance would be substantially identical to that of FIG. 1. As in the description with regard to FIG. 1, a cermet material 31 has been deposited over the surface of the annular rings 26' through 30' for essentially the same purpose as previously described. In the instance of the construction of FIG. 4, it can be seen that the inner cylindrical tube 33 terminates beyond the outermost edge of tube 34. Both tubes 33 and 34 have been shown in part only and would actually extend in the same manner as shown in FIG. 3 to an opposite end of substantially the same construction as the end shown, or have a flat end configuration in accordance with FIGS. 1 and 2. The inner tube 33 would have an opening 15 at each end thereof, as in the case of the description with regard to FIG. 3.

The advantage of the construction offered by spacer 32 is in its use when utilizing the electron analyzer in conjunction with external sources of activating energy for the sample. For example, if one wishes to irradiate sample 20 with X-rays, the frusto-conical configuration of spacer 32 would permit close positioning of an X-ray source 35 adjacent to the sample.

I claim:

1. An end disc for minimization of electrical field fringing in a cylindrical analyzer for charged particles comprising:

a. a generally annular shaped disc member constructed of an electrically insulating material;

b. a high electrical resistance electrically conductive material coating a side surface of said disc member; and

c. a plurality of spaced narrow width annular conductors in physical and electrical contact with said high resistance material and coaxially arranged with respect to the axis of said disc member and with respect to one another.

2. An end disc in accordance with claim 1 wherein said disc member is a flat disc.

3. An end disc in accordance with claim 1 wherein said disc is frusto-conical in shape and said coating-is on the interior side face thereof.

4. An end disc in accordance with claim 1 wherein said disc is alumina and said coating has a resistance of from about to about 100 megohms measured between the inner and outer edges of said disc.

5. An end disc in accordance with claim 1 wherein said annular conductors are metal.

6. An end disc in accordance with claim 5 wherein said conductors are sputtered gold-chromium.

7. A concentric tube electron analyzer comprising:

a. first and second electrically conducting tube members coaxially arranged with respect to one another, the inner of said tubes having openings around the circumference near each end thereof for passage of the electrons being analyzed into the space between the inner and outer tubes;

b. annularly shaped end discs intermediate said tubes adjacent each end of said tubes, said disc being formed of an insulator material and coated on the side interior facing surface thereof with an electrically conductive high electrical resistance material, said electrically conductive material being in electrical contact at its inner and outer edges with the inner and outer tubes respectively;

c. each of said end discs including a plurality of spaced narrow width annular conductors on the inner side thereof in electrical and physical contact with said high resistance coating and coaxially arranged with respect to the axis of each of said discs;

(1. means for applying a potential across said tubes;

e. means for irradiating a sample to be analyzed with high energy radiation when the sample is positioned near the opening defined by a first end of said inner tube to thereby produce secondary electron emissions from said sample; and

f. detecting means at the opposite end of said inner tube for detecting the electrons analyzed by the detector.

8. An analyzer in accordance with claim 7 wherein said annular discs are both planar.

9. An analyzer in accordance with claim 8 wherein said coating has a resistance of from about 10 to about megohms measured between the inner and outer edges of said disc.

10. An analyzer in accordance with claim 7 wherein the annular disc on said first end is of a hollow frustoconical configuration.

l 1. An analyzer in accordance with claim 10 wherein said coating has a resistance of from about 10 to about 100 megohms measured between the inner and outer edges of said disc.

12. An analyzer in accordance with claim 7 wherein said high resistance coating is a cermet having a resistance of about 30 megohms measured between the inner and outer edges of said disc and said low resistance conductors are metal.

13. An analyzer in accordance with claim 12 wherein said conductors are sputtered gold-chromium.

14. An analyzer is accordance with claim 12 wherein said cermet is a sputtered deposit of alumina and nickel 

1. An end disc for minimization of electrical field fringing in a cylindrical analyzer for charged particles comprising: a. a generally annular shaped disc member constructed of an electrically insulating material; b. a high electrical resistance electrically conductive material coating a side surface of said disc member; and c. a plurality of spaced narrow width annular conductors in physical and electrical contact with said high resistance material and coaxially arranged with respect to the axis of said disc member and with respect to one another.
 2. An end disc in accordance with claim 1 wherein said disc member is a flat disc.
 3. An end disc in accordance with claim 1 wherein said disc is frusto-conical in shape and said coating is on the interior side face thereof.
 4. An end disc in accordance with claim 1 wherein said disc is alumina and said coating has a resistance of from about 10 to about 100 megohms measured between the inner and outer edges of said disc.
 5. An end disc in accordance with claim 1 wherein said annular conductors are metal.
 6. An end disc in accordance with claim 5 wherein said conductors are sputtered gold-chromium.
 7. A concentric tubE electron analyzer comprising: a. first and second electrically conducting tube members coaxially arranged with respect to one another, the inner of said tubes having openings around the circumference near each end thereof for passage of the electrons being analyzed into the space between the inner and outer tubes; b. annularly shaped end discs intermediate said tubes adjacent each end of said tubes, said disc being formed of an insulator material and coated on the side interior facing surface thereof with an electrically conductive high electrical resistance material, said electrically conductive material being in electrical contact at its inner and outer edges with the inner and outer tubes respectively; c. each of said end discs including a plurality of spaced narrow width annular conductors on the inner side thereof in electrical and physical contact with said high resistance coating and coaxially arranged with respect to the axis of each of said discs; d. means for applying a potential across said tubes; e. means for irradiating a sample to be analyzed with high energy radiation when the sample is positioned near the opening defined by a first end of said inner tube to thereby produce secondary electron emissions from said sample; and f. detecting means at the opposite end of said inner tube for detecting the electrons analyzed by the detector.
 8. An analyzer in accordance with claim 7 wherein said annular discs are both planar.
 9. An analyzer in accordance with claim 8 wherein said coating has a resistance of from about 10 to about 100 megohms measured between the inner and outer edges of said disc.
 10. An analyzer in accordance with claim 7 wherein the annular disc on said first end is of a hollow frusto-conical configuration.
 11. An analyzer in accordance with claim 10 wherein said coating has a resistance of from about 10 to about 100 megohms measured between the inner and outer edges of said disc.
 12. An analyzer in accordance with claim 7 wherein said high resistance coating is a cermet having a resistance of about 30 megohms measured between the inner and outer edges of said disc and said low resistance conductors are metal.
 13. An analyzer in accordance with claim 12 wherein said conductors are sputtered gold-chromium.
 14. An analyzer is accordance with claim 12 wherein said cermet is a sputtered deposit of alumina and nickel metal. 